If you're looking for isolation and comfort then Shure SE215 and Etymotic Research HF2/3/5 (whichever is the cheapest for you, they are the same IEM with different microphone button control things and the HF5 is without) works out best.
I've not used any Etymotic Research IEMs but I know for sure they isolate the best with only customs beating them. Some people say they are slightly intrusive with the triple flanges though but as I said, I've not tried them so can't comment.
If you get these two then you probably won't need custom moulded tips unless you have serious problems with IEMs.
I know you said that you're not too fussy about sound signatures but their sound signatures are rather different. The Shure SE215 is not very detailed (I've been listening to the RE0 too long) but it has a nice mid centric sound with some nice sub bass and bass. From what I know about the Etymotics, they are known to be very analytical with quite a lot of detail/clarity.
Personally I'm also looking for isolating IEMs for the London Underground too and although the Shure SE215 isolate very very well, I don't like the sound signature so I haven't been using them.
I've been looking at the Minerva Mi-1 customs for isolation and detail but I've also been hearing great things about the Vsonic GR07 but with those two choices, it's a compromise between isolation for better SQ. The Minerva Mi-1 customs will set you back £175 though whereas the Vsonic GR07 is £99 (this one is probably not one to consider though, it's isolation is only average). The Shures are a tad overpriced here compared to the US, they are around £80 the last time I looked. The HF2 is pretty cheap and can be had for about £65 so you may want to try them.